FIG. 1 is a block diagram of a conventional pulsed EMI metal detector and method of operation. A current loop transmitter 10 is placed in the vicinity of the buried metal target 12, and a steady current flows in the transmitter 10 for a sufficiently long time to allow turn-on transients in the soil (soil eddy currents) to dissipate. The transmitter loop current is then turned off. The transmitter current is typically a pulsed waveform with a triangular-ramp or square-wave shape with positive and/or negative current flow.
According to Faraday's Law, the collapsing magnetic field induces an electromotive force (EMF) in nearby conductors, such as the metal target 12. This EMF induces eddy currents to flow in the conductor. Because there is no energy to sustain the eddy currents, they begin to decrease with a characteristic decay time that depends on the size, shape, and electrical and magnetic properties of the conductor. The decay currents generate a secondary magnetic field that is detected by a magnetic field receiver 14 located above the ground and coupled to the transmitter 10 via a data acquisition and control system 16.
Pulse induction metal detector (PIMD) antennas (transmitter and receiver coil) come in two basic types as shown in FIGS. 2a and 2b. The first type of PIMD uses a single transmit and receiver coil 22 with multiple loops of wire forming the coil (FIG. 2a). A current pulse is sent through the multiple turn coil 22 and the received metal detection signal is sensed by the same coil 22. The small voltage generated by the metal target is typically amplified by a high gain electronic amplifier 25 (typical gain factor of 100 to 1000). A protection circuit is provided to protect the sensitive amplifier from the high kick-back voltage pulse generated by switching the inductive coil off abruptly (V=L di/dt, where L is the inductance of the transmitter coil and di/dt is the slope of the current decay in the coil).
The second type of PIMD uses a separate transmitter coil 23 and receiver coil 24, again, with multiple loops of wire forming the coils (FIG. 2b). This configuration provides isolation between the transmitter circuit and the receiver circuit and allows for more flexibility in the receiver coil 24 (e.g., different number of turns, size or differential coil configuration) and amplifier circuit design (e.g., single ended operation of electronics). The high gain amplifier 25 also sees the high kick-back voltage pulse generated by switching the transmitter coil 23 off abruptly and protection circuitry is needed to protect it from damage.
The induced eddy currents in a metal target are proportional to the change in magnetic field with time (ΔB/Δt) at the metal target location. For high sensitivity, one would like to have ΔB (the change in magnetic field) as large as possible and Δt (the change in time) matched to the metal object's time response. For a small metal object with a fast time response the optimal detector sensitivity would be achieved with a small Δt matched to the small metal objects response. For a large metal object with a slower time response the optimal detector sensitivity would be achieved with a larger Δt matched to the metal object's time response. The magnetic field (B) is proportional to the current (I) in the transmitter coil and the number of coil turns (N), thus B˜IN. More coil turns (N) increases the magnetic field at the target depth for a fixed current. However, increasing the number of coil turns also increases the kick-back voltage across the transmitter coil and switch due to the increased inductance. The voltage across the transmitter coil and the electronic switch turning off the coil current is V=L ΔI/Δt and L˜N2.
The desired sensitivity of the metal detector for a small metal object, which needs a small Δt, needs to be balanced with the kick-back voltage at the coil, the available electronic switch voltage rating and wire insulation rating. This is typically done by limiting the coil inductance which lowers the metal detector's sensitivity due to fewer coil turns or smaller coil size and/or increasing the turn-off time.
Several issues arise when attempting to increase a pulse inductive metal detector's sensitivity to small and large metal objects via increasing the ΔB/Δt of the metal detector (i.e., kick-back voltage across the coil). The high voltage electronic components required for the task are expensive, prone to failure and difficult to package. Moreover, the supporting mechanical structure for the coil and components must withstand the high voltage and thus may be expensive or dangerous in some situations (e.g., underwater or explosive gas environments). In addition, high voltage spikes may violate electromagnetic interference emission standards. Lastly, the protection circuit for the receiver electronics must be designed to handle the increased kick-back voltage.
Prior art metal detectors do not address these issues. Instead, prior art pulse inductive metal detectors avoid using high ΔB/Δt dB/dt configurations in their electronics to improve metal detection sensitivity.